In the semiconductor industry, critical steps in the production of integrated circuits are the selective formation and removal of films on an underlying substrate. The films are made from a variety of substances, and can be conductive (for example metal or a magnetic ferrous conductive material) or non-conductive (for example an insulator). Conductive films are typically used for wiring or wiring connections. Non-conductive or dielectric films are used in several areas, for example as interlevel dielectrics between layers of metallization, or as isolations between adjacent circuit elements.
Typical processing steps involve: (1) depositing a film, (2) patterning areas of the film using lithography and etching, (3) depositing a film which fills the etched areas, and (4) planarizing the structure by etching or chemical-mechanical polishing (CMP). Films are formed on a substrate by a variety of well-known methods, for example physical vapor deposition (PVD) by sputtering or evaporation, chemical vapor deposition (CVD), and plasma enhanced chemical vapor deposition (PECVD). Films are removed by any of several well-known methods, for example CMP, dry etching such as reactive ion etching (RIE), wet etching, electrochemical etching, vapor etching, and spray etching.
It is extremely important with removal of films to stop the process when the correct thickness has been removed (the endpoint has been reached). With CMP, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or rotating the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. Overpolishing (removing too much) of a film results in yield loss, and underpolishing (removing too little) requires costly rework (redoing the CMP process). Various methods have been employed to detect when the desired endpoint for removal has been reached, and the polishing should be stopped.
The prior art methods for CMP endpoint detection suitable for all types of films involve the following types of measurement: (1) simple timing, (2) friction by motor current, (3) capacitive, (4) optical, (5) acoustical, and (6) conductive.
An exception to the above is U.S. Pat. No. 5,399,234 to Yu et al, in which a chemical reaction is described between potassium hydroxide in the polishing slurry and the layer being polished. The endpoint for polishing is monitored by sending acoustic waves through the slurry and detecting changes in the acoustic velocity as the concentration of reaction product (thought to be silanol in the case of polishing silicon dioxide) from the layer being polished decreases upon reaching an underlying polish stop layer.
These prior art methods each have inherent disadvantages such as inability for real-time monitoring, the need to remove the wafer from the process apparatus for examining the completion of polishing (not in-situ), or a lack of sensitivity.
These disadvantages have been overcome with an in-situ endpoint detection scheme for conductive films as described in U.S. Pat. No. 5,559,428 to Li et al titled "In-Situ Monitoring of the Change in Thickness of Films," however a suitable endpoint detection for non-conductive films has yet to be described.
Thus, there remains a need for an in-situ, real-time endpoint detection scheme suitable for use with all types of films. Such a scheme should have high detection sensitivity and extremely fast response time, preferably less than 1 or 2 seconds.